WO98/51255 discloses a like ultrasound applying skin care device having an applicator head for generating and transmitting an ultrasound to a user's skin. The device includes a load detecting circuit for detection whether the applicator head is loaded by contact with the skin in order to save energy when the applicator head is out of contact with the skin as well as to apply the ultrasound safely when the applicator head is in contact with the skin. For judging whether the applicator head is loaded or unloaded, the device relies on an electrically equivalent impedance of the applicator head which varies with the load acting on the applicator head, and compares a corresponding voltage being applied to the applicator head with a reference voltage. However, when the applicator head gives the ultrasound at a relatively high frequency, for example, several MHz or more, it may be sometimes difficult to discriminate the unloaded condition from the loaded condition on a basis of the electrically equivalent impedance due to an increased effect of a parasitic resonance appearing in the applicator head.
Japanese Patent Publication No. 7-59197 discloses an ultrasonic vibrator element that reduces vibrations occurring around the periphery of the vibrator element. The vibrator element is in the form of a circular disk provided on opposite surfaces thereof respectively with upper and lower circular electrodes. Each of the electrodes has a diameter less than that of the vibrator element to leave the outer periphery of the vibrator element uncovered in an attempt to cancel undesired vibrations propagating in radial directions, thereby allowing intended ultrasonic vibrations to proceed in the thickness direction of the vibrator element.
The parasitic resonance may be reduced with the use of the structure disclosed in the Japanese Patent Publication No. 7-59197 to some extent. After studying the behavior of the ultrasonic vibrations at the high frequency, the inventors have found effective to restrain vibrations occurring at the center of the applicator head in order to reduce undesired parasitic resonance to such an extent that the applicator head can exhibit the electrically equivalent impedances, respectively when being loaded and unloaded, that are sufficiently distinct from each other for easy discrimination between the loaded and unloaded condition.